Prior to standardization to the 50 Ohm characteristic impedance for RF and microwave components and test equipment, characteristic impedances of components and systems were chosen based on the application. For instance, for high power applications 30 to 40 Ohms was chosen for coaxial transmission lines to accommodate maximum power handling. If an application required minimal losses from the transmission lines, then a 93 Ohm transmission line utilizing an air dielectric proved to work well. For early telecommunication systems 77 Ohm characteristic impedance was chosen for Teflon filled coaxial cables to provide for lowest loss operation. Depending on the application a characteristic impedance was chosen for the transmission lines and the components connecting to the transmission lines were designed to have an equivalent impedance, or a matching circuit was designed and implemented to transform a component from its own natural impedance to the characteristic impedance of the transmission line.
The proliferation of communication and radar systems developed and used during World War II pointed toward the need for a standard characteristic impedance that could be used for these communication systems to allow for a broad group of companies to develop components such as filters, switches, antennas, amplifiers, etc. to be connected to form a complete communication or RF circuit. 50 Ohms was chosen as this standard for RF and communication systems developed in America during this time, and has since been adopted across most of the World for a wide number of industries. This 50 Ohm standard impedance has been adopted by the RF communication systems being integrated into mobile communications such as cell phones, wireless enabled Tablets and laptops, and M2M (Machine to Machine) applications.
With a 50 Ohm characteristic impedance standard in place, component manufacturers can design and manufacture component to be used in communication systems. The drawback with this approach is that some components such as antennas and power amplifiers tend to have natural device impedances that are substantially different than 50 Ohms. This forces the component designer to impedance match the component such that it presents an impedance close to 50 Ohms. Impedance matching a component can take the form of a lumped component matching circuit comprised of inductors and/or capacitors, or it can be comprised of distributed reactance formed by using sections of transmission line in series or parallel configurations. These sections of transmission lines are often termed “tuning stubs”. The matching circuit will have losses associated with it as well as cost, resulting in reduced efficiency in terms of power transfer through the device along with increased cost and size requirements. The losses incurred in the matching circuit of an antenna will result in reduced efficiency of the radiated signal.
A metric as important as matching circuit losses in antenna design is bandwidth. With the additional frequency band requirements of LTE (Long Term Evolution) in cellular communication systems, current mobile wireless devices are requiring wider frequency bandwidth coverage. When an antenna is designed for a communication system such as a cell phone, the antenna is designed to cover the frequency bands of interest, and the antenna must be designed to present a 50 Ohm impedance to the RF circuitry such as an RFFE (Radio Frequency Front-End) that the antenna is connected to. The 50 Ohm requirement tends to decrease the total bandwidth due to the varying impedance properties of an antenna across wide frequency ranges. Designing a matching circuit to transform impedances of the antenna at low frequencies as well as high frequencies results in an increase in matching circuit losses and a reduction in available bandwidth. The instantaneous bandwidth available from the antenna would increase if the antenna could be operated at its natural impedance. By operating at its natural impedance, matching circuits at the antenna RFFE interface would not be required. More specifically, if the antenna can operate at several different natural impedances, with the natural impedance of each frequency band used during operation, then bandwidth can be optimized and matching circuit losses can be minimized.
For mobile communication devices used in cellular communications, the continued adoption of 4G LTE is causing a continued increase in the number of frequency bands needed to be serviced by the RF radio. Also, the introduction of carrier aggregation, where two or more frequency channels, either within the same frequency band or in different frequency bands, are used for simultaneous communication is requiring different topologies for the RFFE. To simplify integration of a wide number of frequency bands and to improve carrier aggregation performance, RFFE architectures are being proposed and implemented wherein a single wide band antenna with a single feed port is being replaced with multiple antennas or a multi-feed antenna. Using multiple antennas where one antenna is used to service the low band frequencies such as 3G and 4G frequencies in the 698 to 960 MHz. range, a second antenna is used to service mid-band frequencies in the range of 1710 to 2170 MHz., and a third antenna is used to service high band frequencies in the range of 2300 to 2700 MHz. allows for each antenna to be better optimized for the frequency bands of interest. Part of this optimization can be associated with the matching circuit to impedance match the antenna to a 50 Ohm characteristic impedance. By restricting the frequency range of the antenna a matching circuit can typically be developed using less components and/or incurring less insertion loss compared to a multi-band antenna. However, if the antenna could be operated at its natural frequency the matching circuit would not be required, reducing insertion loss, cost, and space required for the matching circuit.